Well cementing is a process used in penetrating subterranean zones (also known as subterranean formations) to recover subterranean resources such as gas, oil, minerals, and water. In well cementing, a well bore is drilled while a drilling fluid is circulated through the well bore. The circulation of the drilling fluid is then terminated, and a string of pipe, e.g., casing, is run in the well bore. The drilling fluid in the well bore is conditioned by circulating it downwardly through the interior of the pipe and upwardly through the annulus, which is located between the exterior of the pipe and the walls of the well bore. Next, primary cementing is typically performed whereby a slurry of cement and water is placed in the annulus and permitted to set into a hard mass (i.e., sheath) to thereby attach the string of pipe to the walls of the well bore and seal the annulus.
Low density or lightweight cement compositions are commonly used in wells that extend through weak subterranean formations to reduce the hydrostatic pressure exerted by the cement column on the weak formation. Conventional lightweight cement compositions are made by adding more water to reduce the slurry density. Other materials such as bentonite, diatomaceous earth, and sodium metasilicate may be added to prevent the solids in the slurry from separating when the water is added. Unfortunately, this method has the drawback that the addition of more water increases the cure time and reduces the strength of the resulting cement.
Lightweight cement compositions containing hollow spheres have been developed as a better alternative to the cement compositions containing large quantities of water. The hollow spheres are typically cenospheres, glass hollow spheres, or ceramic hollow spheres. Cenospheres are hollow spheres primarily comprising silica (SiO2) and alumina (Al2O3) and are filled with gas. Cenospheres are a naturally occurring by-product of the burning process of a coal-fired power plant. Their size may vary from about 10 to 350 μm. These hollow spheres reduce the density of the cement composition such that less water is required to form the cement composition. The curing time of the cement composition is therefore reduced. Further, the resulting cement has superior mechanical properties as compared to cement formed by adding more water. For example, the tensile and compressive strengths of the cement are greater.
During the life of the well, the cement sheath is subjected to detrimental cyclical stresses due to pressure and temperature changes resulting from operations such as pressure testing, drilling, fracturing, cementing, and remedial operations. Conventional hollow spheres suffer from the drawback of being brittle and fragile and thus often cannot sustain those cyclical stresses. As a result, the cement sheath develops cracks and thus fails to provide zonal isolation for the life of the well. A need therefore exists to develop a less brittle cement having properties that would enable it to withstand pressure and temperature fluctuations for the life of the well. The present invention advantageously provides cement compositions that can withstand the cyclical stresses that occur during the life of the well.